Onyx
is a banded variety of the oxide mineralchalcedony.
Agate
and onyx are both varieties of layered chalcedony that differ only in the form of the bands: agate has curved bands and onyx has parallel bands. The colors of its bands range from white to almost every color (save some shades, such as purple or blue). Commonly, specimens of onyx contain bands of black and/or white.[3]

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Onyx
comes through Latin (of the same spelling), from the Greek ὄνυξ, meaning "claw" or "fingernail". With its fleshtone color, onyx can be said to resemble a fingernail. The English word "nail" is cognate
with the Greek word.[4]

Onyx is formed of bands of chalcedony in alternating colors. It is
cryptocrystalline, consisting of fine intergrowths of the
silica
minerals quartz
and moganite. Its bands are parallel to one another, as opposed to the more chaotic banding that often occurs in agates.[5]

Sardonyx
is a variant in which the colored bands are sard
(shades of red) rather than black. Black onyx is perhaps the most famous variety, but is not as common as onyx with colored bands. Artificial treatments have been used since ancient times to produce both the black color in "black onyx" and the reds and yellows in sardonyx. Most "black onyx" on the market is artificially colored.[6][7]

The name has sometimes been used, incorrectly, to label other banded
lapidary
materials, such as banded calcite
found in Mexico,
Pakistan, and other places, and often carved, polished and sold. This material is much softer than true onyx, and much more readily available. The majority of carved items sold as "onyx" today are this
carbonate
material.[1][8]

Artificial onyx types have also been produced from common chalcedony and plain agates. The first-century naturalist
Pliny the Elder
described these techniques being used in Roman times.[9]
Treatments for producing black and other colors include soaking or boiling chalcedony in sugar solutions, then treating with sulfuric
or hydrochloric acid
to carbonize sugars which had been absorbed into the top layers of the stone.[7][10]
These techniques are still used, as well as other dyeing treatments, and most so-called "black onyx" sold is artificially treated.[11]
In addition to dye treatments, heating and treatment with nitric acid
have been used to lighten or eliminate undesirable colors.[7]

Onyx is a gemstone found in various regions of the world including Yemen, Uruguay, Argentina, Australia, Brazil, Canada, China, Czech Republic, Germany, India, Indonesia, Pakistan, Madagascar, Latin America, the UK, and various states in the US.[1]

It has a long history of use for
hardstone carving
and jewelry, where it is usually cut as a
cabochon
or into beads. It has also been used for intaglio
and hardstone cameoengraved gems, where the bands make the image contrast with the ground.[12]
Some onyx is natural but much of the material in commerce is produced by the staining of agate.[13]

Onyx was used in
Egypt
as early as the Second Dynasty to make bowls and other pottery items.[14]
Use of sardonyx appears in the art of MinoanCrete, notably from the archaeological recoveries at
Knossos.[15]

Brazilian green onyx was often used as plinths for
art deco
sculptures created in the 1920s and 1930s. The German sculptor Ferdinand Preiss
used Brazilian green onyx for the base on the majority of his chryselephantine
sculptures.[16]
Green onyx was also used for trays and pin dishes – produced mainly in Austria
– often with small bronze animals or figures attached.[17]

Onyx is mentioned in the Bible many times.[18]
Sardonyx (onyx in which white layers alternate with sard) is mentioned in the Bible as well.[19]

The ancient Romans entered battle carrying amulets of sardonyx engraved with
Mars, the god of war. This was believed to bestow courage in battle. In Renaissance Europe, wearing sardonyx was believed to bestow eloquence.[22]
A traditional Persian
belief is that it helped with epilepsy.[23]
Sardonyx was traditionally used by English midwives to ease childbirth by laying it between the breasts of the mother.[24]